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a vacuum-stabilized plastic film wall-warmer
2 nov 1996
our local paper, the collegeville independent, ("accept and defend the truth
whatever the source!") has a lovely 32' long x 20' high south-facing wall with
about 120 ft^2 of windows. last june they wanted to save on the order of
300 btu/hr/ft^2 (peak sun) x 120 ft^2 = 36k btu/hr of peak ac (7 window acs)
by covering the wall with a $130 piece of pak greenhouse shadecloth, with a
perimeter hem and grommets every 4', available from stuppy at (800) 977-5025.
the shadecloth was to be barn red, to match the building. there was a mill
delay, but it should be here soon. this winter we may try a vacuum-stabilized
wall-warmer for solar heating, yielding about 750 btu/ft^2/day max in december,
or the heat equivalent of about 5 gallons of oil. the current plan is: 

1. put a 2x4 frame on edge around the wall perimeter, seal the inside edge
   with a can of spray foam caulk, and paint it red.
2. screw 9 hooks inside the upper frame 2.5" away from the wall every 4', and
   hang a 32' wide x 20' tall piece of 57% red shadecloth on the hooks.
3. install window fans in 1 or 2 upstairs windows, with controls to turn them
   on if it is windy or the sun is shining and the building needs heat.
4. add transparent motorized dampers with honeywell 6161b1000 $50 2 watt motors
   to 1 or 2 lower windows, open when there is sun and office heat is needed.
5. screw 9 more hooks on 4' centers inside the lower frame, 1" away from the
   wall, and attach the bottom edge of the shadecloth to the lower hooks.
6. attach 7 small pulleys under the eave, and hang 40' of #3 (1/8") nylon
   twine from each of 7 upper hooks, starting 4' from the end of the wall.
   later, thread the ends of the twine through 7 holes in the top of the wall,
   into the building (or alternatively, use 60' of twine for each hook, and
   run the ends of the twine back down to ground level cleats on the outside.) 
7. staple a $32, 32' wide x 20' tall piece of 6 mil uv polyethylene film onto
   the face of the frame at the top, and screw on 32' of 1x3 cedar cap strip.
8. attach 20' of cedar 1x3 cap strips over the poly on the south edges of the
   vertical sides of the frame, screwed in place, with hinges so the strips
   can fold out and away from the frame to the east and west, for summer. 
9. make a 32' cedar 1x3 sandwich for the bottom of the film, and screw it onto
   the bottom 2x4, letting the 7 20' or 40' pieces of twine hang out below.  
10.thread the twine through the pulleys and into the building or back down
   to the bottom of the frame. 

it would look like this from the west, not to scale:

           |            3.5"            |

.  pulley . . . . . .  inside twine  . . . . . . > .  ___
o  .       | hook                       |   
u  .       | . \       ==>             fan ==>
t  .       | .   s                      |   
s  .       p .     h                    |   
i  .       o .       a                  |                     office
d  ^       l .         d                |   
e  .       y .           e              |             20'
.  .       | .             c            |   
t  .       f .               l          |   
w  .       i .                 o        |   
i  .       l .                   t      |   
n  .       m .         <==         h    |   
e  .       | .                       \ damper <==
.  . < . . | . _ _ _ _ 2 x 4 _ _ _ _hook|             ___
           
operation:

in the winter, when the sun is shining and the building needs heat, the
damper(s) open and fan(s) turn on to move air out from the building into the
"sunspace" at the bottom, through the shadecloth and back into the building
at the top. when it is windy, and the building does not need heat, or there
is no sun, the fan(s) turn on with the dampers closed, to make a vacuum of
about 0.25" h20 (about 0.01 psi or 1 psf) to pull the poly film in against
the twine inside the frame and keep it from flexing and fatiguing in the wind.
the outside twines help prevent the wind from making the film balloon out.

in the summer, unscrew the bottom 1x3 poly sandwich and fold the cedar cap
strips on the side away from the frame, and roll up the poly film until it
rests under the eave, with the sandwich supported by the u-shaped twines,
to the poly film last longer and allow the shadecloth keep the wall cooler. 

the wall itself needs to be made fairly airtight to make this work.

nick



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